1. Technical Field
This invention relates generally to measuring a parameter of a fluid and more particularly to a method and apparatus for measuring a parameter of a fluid such as velocity and volumetric flow rate of the flow within a pipe.
2. Description of Related Art
A fluid flow process (flow process) includes any process that involves the flow of fluid through pipes, ducts, or other conduits, as well as through fluid control devices such as pumps, valves, orifices, heat exchangers, and the like. Flow processes are found in many different industries such as the oil and gas industry, refining, food and beverage industry, chemical and petrochemical industry, pulp and paper industry, power generation, pharmaceutical industry, and water and wastewater treatment industry. The fluid within the flow process may be a single phase fluid (e.g., gas, liquid or liquid/liquid mixture) and/or a multi-phase mixture (e.g. paper and pulp slurries or other solid/liquid mixtures). The multi-phase mixture may be a two-phase liquid/gas mixture, a solid/gas mixture or a solid/liquid mixture, gas entrained liquid or a three-phase mixture.
Currently, various sensing technologies exist for measuring various physical parameters of fluids in an industrial flow process. Such physical parameters may include, for example, velocity, volumetric flow rate, composition, gas volume fraction, consistency, density, and mass flow rate. One such sensing technology is described in U.S. Pat. No. 6,609,069 to Gysling, entitled “Method and Apparatus for Determining the Flow Velocity Within a Pipe”, and U.S. Pat. No. 6,889,562, which are hereby incorporated herein by reference in their entirety. The '069 patent describes a method and corresponding apparatus for measuring the flow velocity of a fluid in an elongated body (pipe) by sensing vortical disturbances convecting with the fluid. The method includes the steps of providing an array of at least two sensors disposed at predetermined locations along the elongated body, wherein each sensor samples the pressure of the fluid at the position of the sensor at a predetermined sampling rate. The sampled data from each sensor at each of a number of instants of time spanning a predetermined sampling duration is accumulated and at least a portion of a so called k-ω plot is constructed from the accumulated sampled data, wherein the k-ω plot is indicative of a dispersion relation for the propagation of acoustic pressures emanating from the vortical disturbances. A convective ridge in the k-ω plot is identified and the orientation of the convective ridge in the k-ω plot is determined. The flow velocity based on a predetermined correlation of the flow velocity with the slope of the convective ridge of the k-ω plot may then be determined from this information. See also related technology disclosed in U.S. Pat. Nos. 7,673,524 and 7,895,903, which are hereby incorporated by reference.
For certain applications two speed of sound (SOS) measurements may be required on the materials within a pipe to perform a calculation or derive certain characteristics about the materials. An example would be a concept for a density meter where two SOS measurements are made on a material as it passes between two different pipes or sections of pipe that have different compliances. In this case, two SOS measurements are required with high precision to make an accurate density measurement. Of particular importance is the difference in the sound speed between the two measurements, as with all other parameters known, this is directly proportional to the density.
One of the primary issues with making an accurate SOS measurement is the correction for dispersion. Dispersion can arise from a variety of sources, but typically it can be due to the variance of the materials in the pipe such as varying particle sizes, densities or material mixtures in the pipe. This dispersion can manifest itself in the k-ω plane as a curved ridge.
FIG. 1a shows a typical SOS ridge in the k-ω plane without much dispersion. As can be seen, the ridges are basically straight and in this case a high quality SOS measurement can be made. FIG. 1b shows by the curved line what a ridge may follow in the case of dispersion. As seen, it can be difficult to determine the exact ridge location in the presence of dispersion.